This invention relates to a nondestructive methodology for rapid measurement of reaction components in polymerization reactions. In particular, this method describes Raman spectroscopy for quantitation of diphenylcarbonate (DPC) and bisphenol-A (BPA) in polycarbonate melt polymerization reactions.
The stoichiometry of reaction components, such as monomers, solvents and reaction byproducts, can be manipulated during the course of a reaction to influence the final characteristics of the polymer produced. For example, the stoichiometry of polycarbonate monomers such as diphenylcarbonate (DPC) and bisphenol-A (BPA) is important in the production of high quality melt prepared melt polycarbonate resin. Monomer stoichiometry strongly influences polymerization rate, which, in turn, determines the amount of catalyst added and the amount of Fries rearrangement products formed. In addition, monomer stoichiometry determines final polycarbonate endcap levels. Variability in the DPC/BPA stoichiometry, therefore, directly translates into variability in the polycarbonate product.
Thus, for many reactions, it is necessary to monitor the stoichiometric proportions of the various reactants in order to ensure high quality product. In some cases, this may require numerous measurements over a short period of time.
Conventional techniques for monitoring polymerization reactions generally involve analyzing aliquots from the reaction mixture by methods such as liquid chromatography and/or Fourier Transform IR spectroscopy. These and other methods of laboratory analysis, however, are often time consuming, generate additional waste, and for high temperature or high pressure reactions, sampling of materials for laboratory analysis can be dangerous. Also, removing aliquots may alter the reaction conditions or sample constitution, and provides only temporally discrete data points, rather than a continuous profile. Alternatively, samples may be analyzed after the reaction is complete, and unsatisfactory products discarded. Post-reaction sampling, however, does not enable real-time optimization of reaction parameters and, therefore, may result in the synthesis of a polymer batch of substantially inferior quality.
Also, reaction conditions are generally optimized on a smaller scale than used in production. For example, since its introduction in 1970, combinatorial chemistry has become a popular research tool among scientists in many fields. There has been, however, a lag in the development of combinatorial screening for production scale reactions. One reason has been the difficulty in emulating large-scale reactions at the micro-scale necessary for combinatorial work. Another difficulty is that for many reactions, efficient methods of product analysis have yet to be developed. Moreover, methods applied to combinatorial libraries must carry over to analysis of the reaction on a commercial scale.
Therefore, there is a need for an on-line method for optimization of production scale polycarbonate synthesis. The method should eliminate the need for direct sampling and allow for the generation of continuous data. Also, the method should enable optimization of the overall melt prepared process and improve plant capability. Similarly, there is a continuing need to evaluate economically superior reactant systems. Thus, the method should be adaptable to combinatorial evaluation of new reactant and catalyst combinations, as well as production-scale reactant systems.
The present invention is directed to a method for monitoring a reaction mixture using Raman spectroscopy. In one aspect, the invention provides a method for monitoring the process of polymer formation comprising irradiating a polymer with substantially monochromatic radiation; collecting a Raman spectrum corresponding to radiation scattered form the irradiated polymer; monitoring at least one wavenumber of the collected Raman spectrum; correlating the collected spectrum to at least one reaction component of interest; and applying a predetermined selection test to determine whether any one of a set of preselected reaction components needs to be adjusted.
In another aspect, the invention provides a method for monitoring the process of polycarbonate formation comprising irradiating a polymer with substantially monochromatic radiation; collecting a Raman spectrum corresponding to radiation scattered from the irradiated polymer; measuring the intensity of at least two preselected Raman bands; correlating the intensity of at least two preselected Raman bands to the stoichiometry of sample diphenylcarbonate (DPC) and bisphenol-A (BPA), and applying a preselected selection test to determine whether the input of DPC and BPA needs to be adjusted. Also included in the present invention are systems for performing the method.
Yet another aspect of the invention is an apparatus for performing methods of the invention comprising a light source which emits substantially monochromatic radiation to irradiate a polymer sample; a probe which transmits light from the light source to irradiate the polymer sample and collects radiation scattered from the irradiated polymer corresponding to a Raman spectrum; and a detector, wherein the detector monitors at least one wavenumber of the collected Raman spectrum which is correlated to at least one reaction component of interest.